This invention relates to the engraving of cylinders commonly used in the gravure printing process, and specifically to engraving apparatus of the general type disclosed, for example, in U.S. Pat. Nos. 2,881,246, 2,874,479, 3,964,382 and 4,013,829. The basic principle of electro-mechanical engraving of a gravure cylinder involves rotating a copper plated cylinder while actuating an electrically driven tool which cuts or engraves cells or lines into the copper surface. The engraved cylinder is normally used in a web type gravure printing press for printing paper, plastic, or metallic film material.
In the gravure printing process, the engraved cylinder is flooded with ink, and a doctor blade wipes off excess ink from the surface so that only the engraved cells contain ink which is transferred to the material being printed. To obtain a high quality print, it is necessary that the cells be very accurately placed or located on the cylinder surface, usually within one to two microns of the desired theoretical location. The depth of the engraved cells must also be accurately controlled since the the depth determines the amount of ink transferred which, in turn, determines the shade of gray in a black-white print. In a color print, the amount of ink transferred to the paper or material is even more critical since three colors are mixed to produce various shades of all possible colors. A slight variation in the desired amount of ink effects not only the darkness of the color but, more importantly, the production of the desired color tone.
In addition to printing newspapers and magazines, the engraved cylinders may also be used for direct or indirect printing of cloth, applying glue, printing of packaging materials for products, and printing of wood grain patterns for making of wall paneling, floor coverings and other surface coverings.
The cutting tool used to engrave the cells is normally a pointed diamond stylus. Other tools made of sapphire, carbide, cobalt steel, etc. may also be used, but generally give shorter life, and due to wear, do not hold as consistent a point as diamond. The tool must make many cells in a cylinder, and therefore, must be operated at a very high speed. For example, in a typical 140 line screen, about 20,000 cells per square inch are required. More than 100 million cells are frequently required for a single large diameter gravure printing cylinder. Even with a forming rate of 3,000 to 4,000 cells per second, several hours of time may be required to engrave a single cylinder. Such a high cell forming rate introduces serious problems of high acceleration forces with resulting torsional and transverse or lateral vibrations. It is also necessary to make rapid transfer from black to white (full cells to no cells) or white to black. This also introduces transients causing serious torsional and transverse vibrations.
In the engraving of a gravure cylinder, the image pattern or copy to be engraved is usually mounted on a copy cylinder, and the copy is optically scanned while the engraving is being performed. However, the copy may be scanned and the corresponding information stored in computer memory, processed, and later used to engrave a cylinder. The engraving machine may be an electro-mechanical engraver which uses a diamond stylus to engrave the cylinder, or the machine may incorporate electronic means such as electron beam or laser for forming the cells within the cylinder. In either machine, a series of cavities and/or lines are engraved into the cylinder surface. The cavities are adapted to carry ink which produces the image on the material being printed. The image may involve either very small images such as printing type requiring very small and well defined lines or pictures requiring very close control of different cylinders for different colored inks needed for close color matching or large images for printing items such as wallpaper.
In electro-mechanical engraving apparatus, there are a number of specific problems. For example, there is a strong tendency for the mechanical stylus holder and its spring support system to "ring" or vibrate rotationally thereby producing "ghost" images which are displaced from the main image. This greatly reduces the quality of the printing, especially of small type, and dampening means must be used in a form which does not introduce hystersis. The cost of sharpening the diamond engraving stylus is also very high due to the necessity for removing the stylus from its holder in order to grind the cutting surfaces.
There are also problems presented by the bearing or bearings commonly used to support the actuating shaft which carries the cutting tool. A bearing introduces friction, unpredictable hysteresis, and uncontrollable lateral or transverse movement of the cutting tool due to bearing looseness. Even with high strength connections between the power actuated driver and the cutting tool, harmonic vibrations are generally present in the coupling shaft and must be avoided or minimized to produce accurate placement of the cells as well as cells with precisely controlled depths. The stylus holder should also be constructed so that the cutting tool or stylus may be conveniently replaced and precisely aligned without extensive adjustment procedures.
Presently used electro-mechanical engraver systems commonly use a round or ball surface on the diamond guide shoe and thereby avoid the need for precision alignment of the shoe. However, when a round shoe surface passes over a deep engraving, the shoe presses into the copper surface and reduces the quality of the first engraving. While it is necessary for the diamond shoe to be adjustable, the adjustment in presently used machines is provided by mounting the diamond shoe on the end of a fine pitch screw. As a ball point diamond wears, it takes on a slight concaved shape. Thus when the diamond shoe support screw is rotated for adjustment, two lines are drawn on the cylindrical copper surface of the gravure cylinder. These double lines are very undesirable as they may carry ink and appear in the copy printed by the engraved cylinder.
Another problem is presented by the engraving operation producing copper chips which are always present in the vicinity of the cutting stylus and the sliding guide shoe. When these chips pass under the shoe, the shoe and engraving head are forced upwardly causing a lesser depth row of cells and, in turn, a line in the final printed material which may be from a fraction of an inch in length to severl inches in length. While force air is often used to clear chips, further means are desirable to avoid chips passing under the guide shoe.
In the scanning of the image of the copy on the scanning side of the engraving system, there is a problem of obtaining the same reading from a white area surrounded by a dark area as from an equally white area where the whole area is white. This is commonly referred to as light tunneling in the copy material. Light enters the paper or photographic copy material, is transmitted laterally in the copy material, and some of the light extends a small fraction of an inch from where it enters. It is not uncommon to have a reading from a small white spot surrounded by black which is 20 to 40% less than the reading from an equally white spot in a white area. Since this reading controls the engraving depth, it is a serious problem in many copy materials. To avoid this problem, it is desirable to have the illumination spot of light as near as possible to the size of the spot being read.
Since the copy material on the copy support cylinder is not always of uniform thickness and/or may contain wrinkles, it is necessary that the illumination and pickup system have a good depth of focus. The pickup or reading system normally has a good depth of focus, for example, 1/16 inch, by having a high f lens number such as f8 to f22. However, since the illumination must come in at a large angle, i.e., 20 to 60 degrees, to avoid direct surface reflections, the illumination effectively comes from a lens with low f numbers such as f1 to f2. It is difficult to obtain a good depth of focus with such an illumination spot.
Presently used electro-mechanical engravers provide for manually raising and lowering the engraving head relative to the cylinder being engraved. However, if the head is not lowered carefully, the diamond stylus may be damaged, and such damage will not become apparent until after engraving the cylinder, resulting in a defective or damaged cylinder. Also in the "step and repeat" process of engraving a cylinder after each engraving "step", there is a period of no engraving while the scanner is being returned to the start position. If the engraving head is not raised during the time of no engraving, the guide shoe of the engraver runs in a single track around the cylinder and may damage the cylinder surface.